Apparatus for driving piezoelectric transformers

ABSTRACT

In apparatus for driving a transducer including a pair of electrodes there are provided a resonance circuit comprised by an inductance and a capacitance connected in parallel and connected across the input electrodes of the transducer, and means for supplying a periodic pulse current of narrow width to the resonance circuit thereby driving the piezoelectric transducer by the energy delivered by the resonance circuit.

United "$131188 Patent 91,

Kawada s41 APPARATUS FOR DRIVING PIEZOELECTRIC TRANSFORMERS I [75] Inventor: Takehiko- Kawada, Yokohama,

Japan [73] Assignee: Denki Onkyo Company, Ltd.,

- Tokyo, Japan [22] Filed: March 18, 1971 211 App]. No.2 125,702

[30] Foreign Application Priority Data March 20, 1970 Japan ..45/23735 [52] US. Cl. ..310/8.l, BIO/9.8, 331/116 [51] Int. Cl. ..H04r 17/00 [58] Field of Search ..310/8, 8.1, 9.8; 333/72; 331/73,1l6,155, 158,163, 37, 8

[56] References Cited UNITED STATES PATENTS 4/1941 Goddard ..310/a1x 10/1964 Schebler ..310/s.2x

Branson ..3 l0/8.1 UX

3,598,909 8/1971 Sasaki ..310/8.1 3,432,691 3/1969' Shoh ..L ..310/8 1 3,117,768 1/1964 Carlin ..3l0/8,1 X 2,852,676 9/1958 Joy ..310/8.1 X 1,866,267 7/1932 Nicolson ..310/8.6 X 2,975,354 3/1961 Rosen ..310/9.8 X 3,435,368 3/1969 Fukuyo et al ..331/37 3,427,568 2/1969 Bath ..331/116 Primary Examiner-J. D. Miller Assistant Examiner-Mark O. Budd AttarneyChittick, Pfund, Birch, Samuels & Gauthier [5 7] ABSTRACT In apparatus for driving a transducer including a pair of electrodes there are provided a resonance circuit comprised by an inductance and a capacitance connected in parallel and connected across the input electrodes of the transducer, and means for supplying a periodic pulse current of narrow width to the resonance circuit thereby driving the piezoelectric transducer by the energy delivered by the resonance circuit.

6 Claims, 5 Drawing Figures PATENTED 2 sum 1 or 2 FIIIIIVIIIIIInIIIIIIIAU A AU U U FIHVMH IMHI IH U U AU AU AU 1 IIIHIIIH n w w w w m M INVENTOR TAKEHIKO KAWADA PATENTEnJAn 2:915 3.708.701

sum 2 or 2 R E m M0 N8 EN @T V/ Du m R CC 5 NE U AL 0 W W N on O 0 Z ESE mu RP P V 5 mwqfi 6% F G w m F E A k fi F P w V/ r V V INVENTOR v TAKEHIKO KAwADA BY G Momma APPARATUS FOR DRIVING PIEZOELECTRIC TRANSFORMERS BACKGROUND OF THE INVENTION This invention relates to apparatus for driving a piezoelectric transducer or transformer for generating a high voltage.

As is well known in the art a piezoelectric transducer comprises a solid rod or annular ring of sintered lead titanate zirconate or the like and the rod or ring is provided with two polarized regions, one formed with input electrodes and the other an output electrode. An AC driving voltage of the same or approximately the same frequency as the mechanical resonance frequency of the rod or ring is impressed across input electrodes to generate a high AC voltage at the output terminal.

Such a piezoelectric transducer can not generate the high voltage at a desired voltage step-up ratio unless it is driven by a driving voltage having the same or approximately the same frequency as its mechanical resonance frequency. Further, it is essential to use a driving voltage of rectangular waveform in order to increase the voltage step-up ratio and to stabilize the frequency of the driving voltage in order to stabilize the voltage step-up ratio. For this reason, the driving apparatus for the piezoelectric transducer are required to satisfy various requirements described above.

SUMMARY OF THE INVENTION It is an object of this invention to provide a new and improved driving apparatus for a piezoelectric transducer that can produce a nearly perfect rectangular wave.

Another object of this invention is to provide a novel driving apparatus for a piezoelectric transducer according to which the transducer can operate at a high voltage step-up ratio.

Further object of this invention is to provide a novel apparatus for driving a piezoelectric transducer which is suitable to use as a high voltage generating device for a television receiver.

According to this invention there is provided apparatus for driving a piezoelectric transducer including a pair of input electrodes, said apparatus comprising a resonance circuit connected across the input electrodes, and means for supplying a periodic pulse current of narrow width to the resonance circuit, the resonance circuit including an inductance and a capacitance which are connected in parallel thereby driving the piezoelectric transducer by the energy delivered by the resonance circuit.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings FIG. 1 shows a connection diagram of one example of the apparatus for driving a piezoelectric transducer constructed according to the principle of this invention FIG. 2 shows waveforms of various elements shown in FIG. 1;

FIG. 3 is a curve showing the relationship between the output voltage V and the frequency f} of the driving pulse utilized to drive the piezoelectric transducer shown in FIG. 1

FIG. 4 shows a connection diagram of a modified embodiment of this invention and FIG. 5 shows waveforms to explain a method of improving the efficiency of transformation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIG. 1 of the accompanying drawing there is shown a piezoelectric transducer 1 including a pair of input electrodes 11 and 12 applied to the opposite surfaces of one end of a ceramic rod and an output electrode 13 applied to the end surface of the other end. The driving voltage for the piezoelectric transducer 1 is generated by a resonance circuit 2 comprising an inductor 21 and a capacitor 22 connected in parallel therewith. One terminal of the resonance circuit 2 is connected to one input electrode 11 whereas the other terminal is connected to the other input electrode 12 via a source of DC, a battery 3, for example. There is provided a switching element 4 which may be a vacuum tube or a transistor. In the example shown, a NPN type transistor is used with its collector electrode connected to the input terminal 11 of the piezoelectric transducer through a diode 5. In this example, the cathode electrode of diode 5 is connected to'the collector electrode of the transistor. Further, an input circuit network 6 comprising an impedance matching transformer, for example, is connected between a source of pulse 7 or a source of a horizontal synchronizing pulse signal where the piezoelectric transducer is used for a television receiver and the base and emitter electrodes of the transistor 4. One of the output terminals of the input circuit network and the emitter electrode of the transistor are connected to the input electrode 12 of the piezoelectric transducer.

The high voltage appearing at the output voltage may be converted into a high voltage direct current by means of any type of rectifier. In this example, use is made of a voltage multiplying type rectifier circuit 8 comprising capacitors 85, 86 and 87 and diodes 81 through 84 which are connected as shown. The output of the rectifier circuit is impressed across a load 9 which corresponds to the anode electrode of the cathode ray tube of a television receiver, for instance. By the reason described above, the piezoelectric transistor 1 is designed to have a mechanical resonance frequency which is the same or substantially the same as the repetitive frequency or its integer multiple of the horizontal synchronizing pulse signal supplied to the input of transistor 4. Where the horizontal synchronizing signal has a frequency of 15,73 KI-I its third harmonic equals above 47 KII and the fourth harmonic about 63 Kl-I,, and the particular resonance frequency of the piezoelectric transfonner which is to be matched with the frequency or its integer multiple of the horizontal synchronizing signal can be readily determined by the proper selection of the configuration or dimension of the piezoelectric transducer. The values of inductor 21 and capacitor 22 of the resonance circuit 2 are selected to cause the same to resonate at a frequency equal to or nearly equal to the mechanical resonance frequency of the piezoelectric transducer. This relationship among the frequencies of the piezoelectric transducer, resonance circuit and the horizontal synchronizing pulse signalis an important factor in order to stably drive the piezoelectric transducer. The purpose of the diode 5 connected between the collector electrode of transistor 4 and the resonance circuit 2 is to prevent the oscillation current generated by resonance circuit 2 from being short circuited by the transistor, particularly by its base and collector electrodes.

The operation of the circuit will be considered hereunder with the aid of waveforms shown in FIG. 2.

When a positive horizontal synchronizing pulse signal V, as shown in FIG. 2a is impressed upon the base electrode of transistor 4 from source 7 through input circuit network 6, the transistor is tuned ON to pass a saw tooth collector current I, as shown in FIG. 2b, through a circuit including battery 3, inductor 21 of the resonance circuit 2, diode 5, collector-emitter electrodes of transistor 4. This collector current rapidly decreases when the horizontal synchronizing pulse signal applied to the base electrode of transistor 4 disappears. Then, the energy that has been stored in inductor 11 is discharged through capacitor 22, thus creating an oscillation in the resonance circuit 2, the oscillation of course attenuating with time. In the absence of diode 5, the negative half cycles of the oscillation are short circuited by the transistor, thus stopping oscillation. Thus, the diode 5 is effective to prevent this short circuiting and provides a damping effect when the transistor is driven. The resonance voltage V, having a sinusoidal waveform is applied across the input terminals 1 1 and 12 of the piezoelectric transducer l. The waveform of this driving voltage is shown by FIG. 2c.

Upon receival of this resonance voltage V,, the piezoelectric transducer 1 will vibrate at its natural mechanical resonance frequency to produce a high voltage sine wave voltage at its output electrode 13 as shown in FIG. 2d. The output voltage is an extremely stable sine wave voltage because the driving voltage V, is derived from the resonance circuit 2. The output voltage is rectified by the rectifier network 8 and is then applied to load 9.

Summarizing the operation of the novel apparatus, a periodic pulse of a narrow width, such as a horizontal synchronizing pulse signal, and having a frequency equal to or nearly equal to the mechanical resonance frequency of the piezoelectric transducer 1 is impressed upon the transistor 4, energy will be stored in the inductor 21 of resonance circuit 2 while the transistor is conducting. During the period in which the pulse is not applied to the transistor the energy stored in inductor 21 is discharged through capacitor 22 thus creating a sine wave oscillation voltage which is applied across the input electrodes of the transducer.

ln this-manner, storing of energy and an oscillation due to discharge there of are attenuately repeated in the resonance circuit 2 to intermittently apply the sine wave voltage across the input electrodes of the transducer, thus producing a stable and greatly stepped-up output voltage as shown in FIG. 2d at the output electrode l3.

. FIG. 3 shows a relationship between the frequency of synchronizing pulse and the output voltage V of the piezoelectric transducer driven by the driving apparatus shown in FIG. 1. The curve plotted in FIG. 3 shows that the output voltage reaches a maximum when the frequency f, of the synchronizing pulse substantiatly coincides with the mechanical resonance frequency of the piezoelectric transducer.

The novel driving apparatus utilizing the resonance circuit described above is suitable for driving a piezoelectric transducer adapted to drive a large load.

In some cases, in the circuit shown in FIG. 1, it is difficult to match the collector current I, flowing through transistor 4 with the impedance at the driving end of the piezoelectric transducer. In such a case, it is advantageous to use a modified driving circuit shown in FIG. 4 in which an inductor 24 of the resonance circuit 2 is provided with an intermediate tap 23, which is connected to the collector electrode of the transistor 4 through diode 5. With this modified circuit, it is possible to readily supply a sufficiently large driving power to the piezoelectric transducer 1.

In each embodiment, the output power of the resonance circuit 2 increases when the inductance of the inductor is decreased and the capacitance of the capacitor is increased. However, the output power is limited by the maximum collector current of the transistor and the maximum current of diode 5.

FIGS. 5a and 5b are waveforms to explain the method of improving the transforming efficiency of the novel apparatus. These curves show the phase relationship between the driving pulse V, for transistor 4 and the output voltage V, from the resonance circuit 2. Upon application of the resonance voltage V, in the same direction as the collector voltage of the transistor 4 as shown in FIG. 5a, transistor 4 will operate with a large collector loss thus decreasing the efficiency of transformation. If the width of the driving pulse V, were selected such that the pulse is impressed at an instant when the resonance voltage V, becomes zero or negative with respect to the collector voltage of the transistor the efficiency of transformation will be improved substantially.

In addition to a television receiver, the novel driving apparatus can also be used for any piezoelectric transducer designed to generate a high voltage.

While the invention has been shown and described in terms of some preferred embodiments thereof it will be clear that many changes and modifications can be made without departing from the true spirit and scope of the invention as defined in the appended claims.

What is claimed is 1. Apparatus for driving a piezoelectric transformer including a pair of input electrodes, said apparatus comprising: a resonance circuit connected across said input electrodes, said resonance circuit including an inductance and a capacitance which are connected in parallel and said resonance circuit having a resonance frequency equal to or substantially equal to the mechanical resonance frequency or its integer multiple of said piezoelectric transformer; means for supplying periodic current pulses to said resonance circuit at a.

frequency equal to or substantially equal to said mechanical frequency of said transformer or an integer multiple thereof and at a frequency equal to or less than said frequency of said resonance circuit to cause said resonance circuit to resonate to drive said transformer, said pulse supplying means comprising a switching element having a control electrode and means for applying a periodic pulse current signal to said control electrode to render said switching element periodically conductive at the frequency of said pulse supplying means; and a diode connected between said switching element and said resonance circuit to prevent said switching element from stopping oscillation of said resonance circuit when said switching element is not conducting.

2. The apparatus according to claim 1 wherein said switching element comprises a transistor.

3. The apparatus according to claim 1 including a source of DC to energize said resonance circuit and wherein said diode is connected between said switching element and said resonance circuit with a polarity forward with respect to the polarity of said source.

4. The apparatus according to claim 1 wherein said inductor of said resonance circuit is provided with an intermediate tap which is connected to said switching element through said diode.

5. The apparatus according to claim 1 wherein said means for applying a periodic pulse current signal to the control electrode of said switching element is adapted to apply each pulse in said signal at a time 

1. Apparatus for driving a piezoelectric transformer including a pair of input electrodes, said apparatus comprising: a resonance circuit connected across said input electrodes, said resonance circuit including an inductance and a capacitance which are connected in parallel and said resonance circuit having a resonance frequency equal to or substantially equal to the mechanical resonance frequency or its integer multiple of said piezoelectric transformer; means for supplying periodic current pulses to said resonance circuit at a frequency equal to or substantially equal to said mechanical frequency of said transformer or an integer multiple thereof and at a frequency equal to or less than said frequency of said resonance circuit to cause said resonance circuit to resonate to drive said transformer, said pulse supplying means comprising a switching element having a control electrode and means for applying a periodic pulse current signal to said control electrode to render said switching element periodically conductive at the frequency of said pulse supplying means; and a diode connected between said switching element and said resonance circuit to prevent said switching element from stopping oscillation of said resonance circuit when said switching element is not conducting.
 2. The apparatus according to claim 1 wherein said switching element comprises a transistor.
 3. The apparatus according to claim 1 including a source of DC to energize said resonance circuit and wherein said diode is connected between said switching element and said resonance circuit with a polarity forward with respect to the polarity of said source.
 4. The apparatus according to claim 1 wherein said inductor of said resonance circuit is provided with an intermediate tap which is connected to said switching element through said diode.
 5. The apparatus according to claim 1 wherein said means for applying a periodic pulse current signal to the control electrode of said switching element is adapted to apply each pulse in said signal at a time when the output of said resonance circuit is zero or of a polarity opposite to that of said pulse.
 6. The apparatus as recited in claim 5 wherein said means for applying a periodic pulse current signal to the control electrode of said switching element controls the width of the pulses in said signal so that the pulses are applied at a time when the output of said resonance circuit is zero or of a polarity opposite to that of said pulse. 